Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating expenses:
Research and development	$ 12,267	$ 7,407	$ 24,713	$ 13,054
General and administrative	4,345	2,140	9,599	4,281
Total operating expenses	16,612	9,547	34,312	17,335
Loss from operations	(16,612)	(9,547)	(34,312)	(17,335)
Other income (expense):
Interest income	565	234	1,222	392
Change in fair value of preferred stock warrant liability		(150)		(162)
Change in fair value of derivative liability		(1,042)		(1,629)
Other expense	(8)	(2)	(68)	(15)
Total other income (expense), net	557	(960)	1,154	(1,414)
Net loss	(16,055)	(10,507)	(33,158)	(18,749)
Comprehensive loss	(16,055)	(10,507)	(33,158)	(18,749)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(16,055)	(10,507)	(33,158)	(18,749)
Accretion of issuance costs on redeemable convertible preferred stock				(2,243)
Net loss attributable to common stockholders—basic and diluted	$ (16,055)	$ (10,507)	$ (33,158)	$ (20,992)
Net loss per share attributable to common stockholders—basic and diluted (Note 10)	$ (0.67)	$ (2.98)	$ (1.38)	$ (7.16)
Weighted-average number of common shares used in computing net loss per share attributable to common stockholders— basic and diluted	24,046,262	3,529,269	23,985,717	2,930,358